The invention relates to a helix resonator. In particular, it relates to a helix resonator comprising a helically wound electrical conductor having a low impedance end and a high impedance end, and a reactive element.
A helix resonator or helix is a transmission line resonator having an electrical length of about a quarter-wave length. It is well known to use helix resonators as tuning elements, and they are widely used in filters in the high frequency range, particularly from 100 to 2000 MHz. Resonators of this kind comprise inductive elements such as an electrical conductor wound into a cylindrical or helical coil, and a metallic cover surrounding the cylindrical coil and spaced a distance away from it. The low impedance end of the coil is earthed and may be connected directly to the metallic cover which is itself earthed.
A possible arrangement for connecting the helical coil to the metallic cover is to have a straight length of conductor at an end of the helical coil and arranged approximately perpendicular to an end face of the resonator cover. The first turn of the helical coil is spaced a distance from the cover determined by the straight length of conductor. The other end of the helical coil is the high impedance end which is spaced away from and capacitively coupled to the cover.
The resonator is electrically connected to the rest of a filter circuit, another electronic circuit or the like by coupling a connecting conductor to the helical coil. From the helical coil the connecting conductor passes through, and is insulated from the cover and is then connected to a circuit. Coupling of the connecting conductor to the resonator may be by means of a solder joint or the like, and the point at which the coupling is made is known as the tapping point. The input impedance to the coil seen by the connecting conductor at the tapping point depends upon its position along the helical coil. By an appropriate choice of tapping point the resonator can be matched to the circuit. The tapping point may be determined experimentally or by theoretical calculations. However, it is generally located at or near the first turn of the helical coil.
The characteristic impedance of the helix resonator is determined by the ratio of the coil diameter and the inner dimension of the cover surrounding it, by the mutual distance between the coil turns or the so called pitch, and by the dielectric material supporting the resonator. The resonance frequency of the helix resonator is a function of the coil's physical dimensions, the capacitive construction and the distance between the high impedance end and the cover. Therefore production of a resonator with a certain frequency range requires exact and accurate construction.
From Finnish patent FI-78198 a helix resonator is disclosed, in which the resonator coil is supported by a dielectric board. A portion of the dielectric board contains an electric circuit formed by strip lines to which the resonator is electrically connected. Means to produce a helix resonator with an exact and reproducible tapping point is disclosed in Finnish patent FI-80542. In FI-80542 there is disclosed a construction which is partly the same as in the resonator of patent FI-78198, but at a certain place on the surface of the dielectric board there is a micro strip conductor, whereby the coil is always connected at the same place to the micro strip when the coil is connected to the microstrip. The micro strip conductor can also be guided directly outside the resonator or it can be connected to the electrical circuit on the dielectric board, which acts as a support as disclosed in Finnish patent FI-78198.
Helix resonators are used in high frequency radio equipment due to their good high frequency characteristics, and especially due to their small size. When several of these resonators are placed close together and connected in a suitable way to a form a working unit it is possible to manufacture a small size high frequency filter with good high frequency characteristics. These filters are widely used in radio equipment, particularly in mobile radio telephones and in radio telephone equipment mounted in cars. As the size of radio equipment decreases, the filter size is also substantially reduced. This requires greater accuracy than previously in the manufacture and assembly of high frequency components, due to a corresponding reduction in tolerances.
The physical lengths of helix resonators used in high frequency filters often differ considerably from each other. In a single filter it is possible that all the resonators have different lengths, which increases the number of different components required to assemble the filter. The increase in the component number can substantially increase the product's manufacturing time, or at least the risk of mix-up between components. A large number of different components impedes the development of automatization and can hinder increased automatization degree in the filter production.